Tiziana Di Salvo

XIPE: the X-ray imaging polarimetry explorer

Abstract X-ray polarimetry, sometimes alone, and sometimes coupled to spectral and temporal variability measurements and to imaging, allows a wealth of physical phenomena in astrophysics to be studied. X-ray polarimetry investigates the acceleration process, for example, including those typical of magnetic reconnection in solar flares, but also emission in the strong magnetic fields of neutron stars and white dwarfs. It detects scattering in asymmetric structures such as accretion disks and columns, and in the so-called molecular torus and ionization cones. In addition, it allows fundamental physics in regimes of gravity and of magnetic field intensity not accessible to experiments on the Earth to be probed. Finally, models that describe fundamental interactions (e.g. quantum gravity and the extension of the Standard Model) can be tested. We describe in this paper the X-ray Imaging Polarimetry Explorer (XIPE), proposed in June 2012 to the first ESA call for a small mission with a launch in 2017. The proposal was, unfortunately, not selected. To be compliant with this schedule, we designed the payload mostly with existing items. The XIPE proposal takes advantage of the completed phase A of POLARIX for an ASI small mission program that was cancelled, but is different in many aspects: the detectors, the presence of a solar flare polarimeter and photometer and the use of a light platform derived by a mass production for a cluster of satellites. XIPE is composed of two out of the three existing JET-X telescopes with two Gas Pixel Detectors (GPD) filled with a He-DME mixture at their focus. Two additional GPDs filled with a 3-bar Ar-DME mixture always face the Sun to detect polarization from solar flares. The Minimum Detectable Polarization of a 1 mCrab source reaches 14 % in the 2–10 keV band in 105 s for pointed observations, and 0.6 % for an X10 class solar flare in the 15–35 keV energy band. The imaging capability is 24 arcsec Half Energy Width (HEW) in a Field of View of 14.7 arcmin × 14.7 arcmin. The spectral resolution is 20 % at 6 keV and the time resolution is 8 μs. The imaging capabilities of the JET-X optics and of the GPD have been demonstrated by a recent calibration campaign at PANTER X-ray test facility of the Max-Planck-Institut für extraterrestrische Physik (MPE, Germany). XIPE takes advantage of a low-earth equatorial orbit with Malindi as down-link station and of a Mission Operation Center (MOC) at INPE (Brazil). The data policy is organized with a Core Program that comprises three months of Science Verification Phase and 25 % of net observing time in the following 2 years. A competitive Guest Observer program covers the remaining 75 % of the net observing time.

Neutron stars with submillisecond periods: A population of high-mass objects?

Rapidly spinning neutron stars, recycled in low-mass binaries, may have accreted a substantial amount of mass. The available relativistic measurements of neutron star masses, all clustering around 1.4 M☉, however, refer mostly to slowly rotating neutron stars that accreted a tiny amount of mass during evolution in a massive binary system. We develop a semianalytical model for studying the evolution of the spin period P of a magnetic neutron star as a function of the baryonic mass load Mac; evolution is followed down to submillisecond periods, and the magnetic field is allowed to decay significantly before the end of recycling. We use different equations of state and include rotational deformation effects and the presence of a strong gravitational field and of a magnetosphere. For the nonmagnetic case, comparison with numerical relativistic codes shows the accuracy of our description. The minimum accreted mass requested to spin up a magnetized 1.35 M☉ neutron star at a few milliseconds is ~0.05 M☉, while this value doubles for an unmagnetized neutron star. Below 1 ms, the request is for at least ~0.25 M☉. Only highly nonconservative scenarios for the binary evolution could prevent the transfer of such a mass to the compact object. Unless a physical mechanism limits the rotational period, there may exist a yet undetected population of massive submillisecond neutron stars. The discovery of a submillisecond neutron star would imply a lower limit for its mass of about 1.7 M☉.

QPO emission from moving hot spots on the surface of neutron stars: a model

We present recent results of 3D magnetohydrodynamic simulations of neutron stars with small misalignment angles, as regards the features in light curves produced by regular movements of the hot spots during accretion on to the star. In particular, we show that the variation of position of the hot spot created by the infalling matter, as observed in 3D simulations, can produce high-frequency quasi-periodic oscillations (QPOs) with frequencies associated with the inner zone of the disc. Previously reported simulations show that the usual assumption of a fixed hot spot near the polar region is valid only for misalignment angles Θ relatively large. Otherwise, two phenomena challenge the assumption: one is the presence of Rayleigh-Taylor instabilities at the disc-magnetospheric boundary, which produce tongues of accreting matter that can reach the star almost anywhere between the equator and the polar region; the other one is the motion of the hot spot around the magnetic pole during stable accretion. In this paper, we start by showing that both phenomena are capable of producing short-term oscillations in the light curves. We then use Monte Carlo techniques to produce model light curves based on the features of the movements observed, and we show that the main features of kHz QPOs can be reproduced. Finally, we show the behaviour of the frequencies of the moving spots as the mass accretion rate changes, and propose a mechanism for the production of double QPO peaks.

X-ray spectroscopy of MXB 1728-34 with XMM-Newton

We analyzed an XMM-Newton observation of the low-mass X-ray binary and atoll source MXB 1728–34. The source was in a lowluminosity state during the XMM-Newton observation, corresponding to a bolometric X-ray luminosity of 5 × 10 36 d 2.1 kpc erg s −1 . The 1–11 keV X-ray spectrum of the source, obtained combining data from all the five instruments on-board XMM-Newton ,i s well fitted by a Comptonized continuum. Evident residuals are present at 6−7 keV, which are ascribed to the presence of a broad iron emission line. This feature can be equally well fitted by a relativistically smeared line or by a self-consistent, relativistically smeared reflection model. Under the hypothesis that the iron line is produced by reflection from the inner accretion disk, we can infer important information on the physical parameters of the system, such as the inner disk radius, Rin = 25−100 km, and the inclination of the system, 44 ◦ < i < 60 ◦ .

Secular spin-down of the AMP XTE J1751-305

Context. Of the 13 known accreting millisecond pulsars, only a few have displayed more than one outburst during the RXTE era. After its main outburst in 2002, XTE J1751-305 showed an additional three dim outbursts. We report on the timing analysis of the latest one, which occurred on October 8, 2009 and was serendipitously observed from its very beginning by RXTE. Aims. By detecting the pulsation during more than one outburst, we derive a stronger constraint of the orbital parameters and their evolution, and we can track the secular spin frequency evolution of the source. Methods. Using the RXTE data of the last outburst of the AMP XTE J1751-305, we performed a timing analysis to more accurately constrain the orbital parameters. Because of the low quality of the data statistics, we applied an epoch-folding search technique to the whole data set to improve the local estimate of the time of ascending node passage. Results. Using this new orbital solution, we epoch-folded data obtaining three pulse phase delays over a time span of 1.2 days, that we fitted using a constant spin frequency model. Comparing this barycentric spin frequency with that of the 2002 outburst, we obtained a secular spin frequency derivative of −0.55(12) × 10 −14 Hz s −1 . We estimate the pulsar’s magnetic dipole value by assuming that the secular spin-down is due to a rotating magneto dipole emission, to be consistent with what is assumed for radio pulsars. We derive an estimate of the magnetic field strength at the polar cap of BPC = 4.0(4) × 10 8 G, for a neutron star mass of 1.4 M� , assuming the Friedman Pandharipande Skyrme equation of state.

Chandra X-ray spectroscopy of a clear dip in GX 13+1

Context. The source GX 13+1 is a persistent, bright Galactic X-ray binary hosting an accreting neutron star. It shows highly ionized absorption features, with a blueshift of ∼400 km s −1 and an outflow-mass rate similar to the accretion rate. Many other X-ray sources exhibit warm absorption features, and they all show periodic dipping behavior at the same time. Recently, a dipping periodicity has also been determined for GX 13+1 using long-term X-ray folded light-curves, leading to a clear identification of one of such periodic dips in an archival Chandra observation. Aims. We give the first spectral characterization of the periodic dip of GX 13+1 found in this archival Chandra observation performed in 2010. Methods. We used Chandra/HETGS data (1.0–10 keV band) and contemporaneous RXTE/PCA data (3.5–25 keV) to analyze the broad-band X-ray spectrum. We adopted different spectral models to describe the continuum emission and used the XSTAR-derived warm absorber component to constrain the highly ionized absorption features. Results. The 1.0−25 keV continuum emission is consistent with a model of soft accretion-disk emission and an optically thick, harder Comptonized component. The dip event, lasting ∼450 s, is spectrally resolved with an increase in the column density of the neutral absorber, while we do not find significant variations in the column density and ionization parameter of the warm absorber with respect to the out-of-dip spectrum. Conclusions. We argue that the very low dipping duty-cycle with respect to other sources of the same class can be ascribed to its long orbital period and the mostly neutral bulge that is relatively small compared with the dimensions of the outer disk radius.

The near-IR counterpart of IGR J17480-2446 in Terzan 5 ⋆

Context. Some globular clusters in our Galaxy are noticeably rich in low-mass X-ray binaries. Terzan 5 has the richest population among globular clusters of X- and radio-pulsars and low-mass X-ray binaries. Aims. The detection and study of optical/IR counterparts of low-mass X-ray binaries is fundamental to characterizing both the lowmass donor in the binary system and investigating the mechanisms of the formation and evolution of this class of objects. We aim at identifying the near-IR counterpart of the 11 Hz pulsar IGRJ17480-2446 discovered in Terzan 5. Methods. Adaptive optics (AO) systems represent the only possibility for studying the very dense environment of GC cores from the ground. We carried out observations of the core of Terzan 5 in the near-IR bands with the ESO-VLT NAOS-CONICA instrument. Results. We present the discovery of the likely counterpart in the KS band and discuss its properties both in outburst and in quiescence. Archival HST observations are used to extend our discussion to the optical bands. Conclusions. The source is located at the blue edge of the turn-off area in the color-magnitude diagram of the cluster. Its luminosity increasefrom quiescence to outburst, by a factor 2.5, allows us to discuss the nature of the donor star in the context of th e double stellar generation population of Terzan 5 by using recent stellar evolution models.

Order in the chaos? The strange case of accreting millisecond pulsars

We review recent results from the X‐ray timing of accreting millisecond pulsars in LMXBs. This is the first time a timing analysis is performed on accreting millisecond pulsars, and for the first time we can obtain information on the behavior of a very fast pulsar subject to accretion torques. We find both spin‐up and spin‐down behaviors, from which, using available models for the accretion torques, we derive information on the mass accretion rate and magnetic field of the neutron star in these systems. We also find that the phase delays behavior as a function of time in these sources is sometimes quite complex and difficult to interpret, since phase shifts, most probably driven by variations of the X‐ray flux, are sometimes present.

Quantum clock: A critical discussion on spacetime

We critically discuss the measure of very short time intervals. By means of a Gedankenexperiment, we describe an ideal clock based on the occurrence of completely random events. Many previous thought experiments have suggested fundamental Planck-scale limits on measurements of distance and time. Here we present a new type of thought experiment, based on a different type of clock, that provide further support for the existence of such limits. We show that the minimum time interval

A possible cyclotron resonance scattering feature near 0.7 keV in X1822-371

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